Operation of alkali metal chlorine cells

ABSTRACT

IMPROVED MERCURY CELL OPERATION IS ACHIEVED BY REMOVING FROM A MERCURY CELL DURING OPERATION A SIDE STREAM OF AMALGAM. AMALGAM REMOVED FROM AN OPERATING MERCURY CELL IS TYPICALLY DENUDED AND THEN SUBJECTED TO ELECTROLYSIS TO CAUSE METAL IMPURITIES IN THE MERCURY RESULTING FROM THE DENUDING OPERATION TO BE PLATED OUT ON THE CATHODE OF THE ELECTROLYTIC CELL USED FOR SUCH ELECTROLYSIS. IN THE ELECTROLYTIC CELL USED FOR PURIFICATION THE MERCURY FORMS THE ANODE OF THE CELL. MERCURY SUBSTANTIALLY DEPLETED OF METAL ION IMPURITITES IS RETUREND TO THE MERCURY CELL FROM WHICH IT WAS REMOVED THEREBY ENHANCING THE OPERATION OF THE MIAN MERCURTY CELL BY THE REMOVAL OF SAID METAL ION IMUPURITIES.

Feb. 9,

W. CARLIN ETAL OPERATION OF ALKALI METAL CHLORINE CELLS Filed April 11, 1967 3 Sheets-Sheet 1 INVENTORS WILL/AM w. CARL/IV CARL. w. EAET'ZSCH ATTORNEY5 Feb. 9, 1971 w. w. CARLIN ETAL 3,562,123

OPERATIONOF ALKALI METAL CHLORINE CELLS Filed April 11. 19s? 5 Sheets-Sheet 2 H OUT m HQ :0 m .04 1, I00 I07] Fl6.l

mvmoks WILL/AM wow/1v 4.. URL WRAETZSCH ATTORNEY;

Feb. 9, 1971 w, w, cARLlN ETAL 3,562,123

OPERATION OF ALKALI METAL CHL ORINE CELLS Filed April ll, 1967 3 Sheets-Sheet s I 2 3 4 5 b 7 8 9 l0 ll l2 l3 l4 I5 lb|7 l8 I9202l2223242526T/Z8 DAYS OPERATION DAYs OPERATION CARL W. EAETzscH A ORNEYg United States Patent Ofifice 3,562,123 OPERATION OF ALCALLISMETAL CHLORINE E L William W. Carlin, Portland, and Carl W. Raetzsch,

Corpus Christi, Tex., assignors to PPG Industries Inc.,

Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 11, 1967, Ser. No. 630,055 Int. Cl. COld 1/08; COlb 11/26 US. Cl. 20499 8 Claims ABSTRACT OF THE DISCLOSURE Improved mercury cell operation is achieved by removing from a mercury cell during operation a side stream of amalgam. Amalgam removed from an operating mercury cell is typically denuded and then subjected to electrolysis to cause metal impurities in the mercury resulting from the denuding operation to be plated out on the cathode of the electrolytic cell used for such electrolysis. In the electrolytic cell used for purification the mercury forms the anode of the cell. Mercury substantially depleted of metal ion impurities is returned to the mercury cell from which it was removed thereby enhancing the operation of the main mercury cell by the removal of said metal ion impurities.

BACKGROUND OF INVENTION The present invention concerns itself with the operation of the alkali metal chlorine cells of the mercury type. The invention is practiced in conjunction with the operation of cells such as those described in US. Patent 3,140,- 991, granted July 14, 1964 and US. Patent 3,271,289, granted Sept. 6, 1966, and other similar cells employing a flowing mercury cathode. In these typifications of mercury cells, a mercury cathode is employed. The anode of the cell may be composed of any suitable electrode material such as graphite, carbon, platinized titanium, copper electrodes brazed with titanium and containing a platinum surface or any other material suitable as an anode material and compatible with the electrolyte. In the operation of these cells, generally speaking, mercury is caused to flow across the cell usually in a horizontal plane and between an electrode member located above it. Electric current is passed from the surface of the anode above the mercury through the electrolyte to the mercury. The electrolysis of the alkali metal chloride solutions results in the discharge of gaseous chlorine at the anode surface and the formation of sodium amalgam on the mercury flowing across the cell. The sodium amalgam is ultimately removed from the cell, denuded or relieved of its sodium content by contacting it with water or by the utilization of other conventional means which form no part of the instant invention. The mercury resulting from the denuding operation is then returned to the cell for further use as a cell cathode.

In the operation of these cells one of the major difficulties encountered by cell operation is the formation of light and heavy butters in the cell. These so-called butters are believed to be caused by the formation of metal hydroxides corresponding to various metal ion impurities present in the mercury. These ions are picked up by mercury as it passes through the electrolytic cell system. Pickup occurs from contact of mercury with the brine and with various metal surfaces encountered in pumping equipment, feed lines and the cell itself. The formation of these butters causes considerable difficulty in operating alkali metal-chlorine mercury cells because they sometimes form heavy sludges on the bottom of the cell trays hindering the fiow of mercury. In addition, at the ends of the cells they quite frequently clog up 3,5fi2,123 Patented Feb. 9, 1971 sumps and render it diflicult to remove amalgam from the cell so that it can be properly denuded. High butters also result in the formation of considerable quantities of hydrogen in operating cells which is a dangerous condition in the operation. The sludge materials also find their way into the denuders and hamper their operation.

SUMMARY OF INVENTION In accordance with the teachings of the present invention these difficulties encountered by the prior art with respect to butter formations in electrolytic alkali metal chlorine cells of the mercury type are eliminated or substantially minimized. In accordance with the teachings of this invention sodium amalgam is removed from alkali metal chlorine cells of the mercury type, denuded and is then passed into an auxiliary cell where it is electrolyzed in the presence of a suitable electrolyte. Conveniently the electrolyte may comprise brine or alkali metal hydroxide solutions though other electrolytes may be employed. The electrolysis of the mercury results in the removal of metal ion impurities present therein by,

dissolution in the electrolyte and some plating out of the metals on the surface of the cathode of the auxiliary cell. In this manner removal of these impurities from the mercury which forms the anode of the auxiliary cell is accomplished. In an operation of this type therefore the sodium amalgam contaminatedwith metal ion impurities removed from an operating alkali metal chlorine mercury cell has its metal ion impurities removed from the mercury component and the purified mercury can be recirculated to the mercury cell from which it was removed. Butter formation problems normally associated with the operation of alkali metal chlorine mercury cells is in this manner eliminated or minimized greatly.

For a more complete understanding of the present invention reference is made to the accompanying drawings in which:

FIG. 1 shows diagrammatically a train involving a mercury cell and its auxiliary clean-up equipment.

FIG. 2 shows a diagrammatic sketch of an auxiliary cleaning cell.

FIG. 3 shows a plurality of horizontal mercury cells connected together and utilized for the purpose of producing chlorine and caustic soda from a brine where the brine is circulated in series through each of the cells.

FIG. 4 shows the operation of the instant invention in conjunction with a large horizontal alkali metal chlorine cell of the mercury type.

FIG. 5 shows a diagrammatic illustration of the electrode elements of a control device for the auxiliary cleaning cell of FIG. 1.

FIG. 6 shows the auxiliary cleaning cell and the associated voltage control devices for maintaining voltages within limits during operation of the cell.

FIG. 7 shows a curve representing the quantity of sludge in an operating mercury cell plotted against days of service where no auxiliary cleaning cell is employed.

FIG. 8 shows another plot of an operating mercury cell of the same type and run under similar conditions as the cell used for the plot of FIG. 8 but with an auxiliary cleaning cell used in association therewith.

Turning to the figures in more detail, in FIG. 1 there is shown an electrolytic cell assembly 1. This cell assembly 1 is of the type described in our co-pending application Ser. No. 410,579, filed Nov. 12, 1964, now abandoned. In this cell assembly a plurality of electrolytic cells are provided in a single unitary housing or box 1. The mercury is individually fed to the cells and is removed from the cells individually. The mercury leaves the housing in a line generally as line 2 on said drawing. In the drawing the box 3 represents a denuder which is utilized to produce sodium hydroxide from the sodium amalgam removed from the cell box 1 in line 2. The sodium hydroxide produced in the denuder 3 is removed via line 13. The mercury from this denuder 3 is passed via line 11 through pump 12 to line 9 where it is returned to the cell assembly 1. A small side stream of mercury is taken from line 11 via line 5 and is passed into an auxiliary cleaning cell 6. In cell 6 the mercury is used as the cell anode and electrolysis is conducted in the presence of a suitable electrolyte, typically caustic. The metal impurities in the mercury are removed in this cell 6 by plating them out in the cell cathode. The purified mercury is then passed out of the cell 6 via line 7 toline 9 for return to the main cell assembly 1.

In the operation of the system shown in FIG. 1 brine is introduced into the cell housing 1 via line 14. The cell is activated, that is, electric power is fed to the cell in quantities and voltages sufficient to cause electrolysis of the alkali metal brine solution to take place. Elemental chlorine generated in the cell is removed via conventional gas lines not shown in the drawing and sodium amalgam is produced. Sodium amalgam is removed via line 2 and passed into the denuder 3. In denuder 3 the sodium amalgam is contacted with water usually in the presence of contact substances such as granulated carbon, etc., and the sodium hydroxide product is removed via line 13. The mercury substantially depleted of its sodium content is removed from the denuder member 3 via line 11 utilizing pump 12 and is passed into line 9 where it is once again returned to the cell.

In the cell assembly 1 of this drawing a plurality of bipolar cells are contained in a unitary housing. These cells are stacked one above the other. A cell of this arrangement is described in our co-pending application Ser. No. 410,579, filed Nov. 12, 1964. The brine in this cell is introduced via line 14 and circulates from the top cell to the bottom cell in series and exits the cell assembly via line 15. During the operation of a cell assembly of the type generally indicated at 1 in the drawing, substantial formation of butter normally occurs after some period of electrolysis. Since the cell trays of an assembly of this type are fairly close together physically, the formation of butters on the tray surface hindering the flow of mercury or causing a frothing at the exit end of the cell where the sodium amalgam product is removed causes considerable mechanical difficulty.

If desired the small stream of mercury fed to cell 6 can be removed as amalgam from the first or second cell trays in housing 1. In this instance it will be understood that the stream should be denuded before it is fed to the cleaning cell 6. Clean-up cell 6 is indicated diagrammatically in FIG. 2 and is composed of cell housing 20. Cell housing is provided with a cathode member 21 suitably connected via lines 22 and 23 to an electric power source not shown in the drawing. The anode of the cell comprises the bottom 24 of the cell housing 20 and the mercury pool 28 electrically associated with or connected to it. The cell bottom 24 is suitably connected via line 25 and 27 to the positive pole of an electric power source. The mercury fed to the unit 6 forms the anode of the cell and is generally indicated at 28 in the drawing. The electrolyte, typically sodium hydroxide is indicated at 29. While caustic is shown in this embodimenh as the electrolyte it is of course to be understood that any electrolyte may be employed in this clean-up cell for the purpose of removing metal ion impurities from the mercury fed thereto. Caustic forms a preferred embodiment since it is completely compatible with a main mercury cell system and is readily available in pure condition in alkali chlorine mercury cell plants. The quantity of the bleed stream fed to the cell 6 can be quite small and may represent as little as 0.5 percent of the total quantity of mercury flowing through the cell assembly 1. Typically this side stream will represent 0.5 to 10 percent of the total mercury flowing through the cell as- 4 sembly. It can comprise even larger quantities of mercury and the exact quantity employed will depend upon the quantity of impurities picked up during the electrolysis or the quantity of butter formation encountered during the operation of the electrolytic cell assembly.

Turning back to FIG. 1, the mercury is passed into the electrolytic cell assembly 1 via line 9. The bleed stream is passed via line 5 into the electrolytic cell where electrolysis occurs. The electrolysis is maintained at a voltage such that no substantial decomposition of the mercury takes place within the cell 6. The mercury is then removed from the cell after its passage through the cleanup area via line 7. This mercury is then passed via line 9 back into cell assembly 1.

Turning now to FIG. 3, there is shown a plurality of horizontal mercury cells 31, 32, 33 and 34. These cells are supplied with a series connected brine system. Brine is thus introduced into cell 31 via line 35. The brine after its passage through the cell 31 is passed via line 36 into cell 32 and from cell 32 the brine is passed via line 37 to cell 33. Similarly the brine as it exits cell 33 is removed via line 38 and introduced into cell 34 where it is ultimately removed substantially depleted of its alkali metal chloride content via line 39.

In an operation of this character the formation of mercury butters in the horizontal trays of these generally elongated mercury cells causes considerable difficulty during cell operation. Normally these cells are provided with a unitary, self-contained denuding system shown in FIG. 4 in more detail. Thus, in FIG. 3 when the brine leaves the cell 31 via line 36 the amalgam is removed via line 40 and is passed to a suitable denuding system. Each of the cells 32, 33 and 34 are also provided with amalgam removal lines 42, 43 and 44, respectively, and these amalgams are likewise fed to a denuder generally associated with the cell from which the amalgam is removed.

Operation of a cell of this type may be described by reference to FIG. 4. As shown in FIG. 4 a cell 41 is provided with a brine inlet 42 and a brine outlet 43. The cell is also provided with a mercury inlet, line 44, and an amalgam outlet, line 45. The denuder of the cell is generally indicated at 46. In this arrangement mercury is removed via line 44 and fed back to the cell after passage through the denuder 46. A small side stream is removed from line 44 via line 50 where it is passed into a denuder 49 and then fed via line 53 to a clean-up cell 48. After the mercury is passed through cell 48 where it is used as a cell anode during electrolysis as hereinbefore described with reference to FIG. 1, it is reintroduced into the cell 41, typically by passing it back to line 44 via line 47. Denuder 49 is provided with a sodium hydroxide removal means 51. Sodium hydroxide produced during the operatsion of cell 41 is removed from the denuder 46 via line In the operation of FIG. 3 as described above, it is also possible to operate a string of cells of the character shown therein in series so that the mercury is fed via line 35 to the first cell 31 and amalgam is introduced via lines 36, 37 and 38 to cells 32, 33 and 34, respectively. In this type of an operation, the present invention has particularly beneficial effects in that the first cell can be utilized for the purpose of providing clean mercury to the remaining cells. Thus, the first cell 31, in the embodiments shown in FIG. 3, can be employed as the butter removal cell and a small bleed stream of mercury can be removed from the cell via dotted line 36 and fed to a denuder 31. The amalgam substantially denuded and depleted, now mercury, can then be fed to a small cleaning cell generally indicated as 40. In this cell electrolysis is taking place with the mercury forming the cell anodes. The purified mercury removed from cell 40" is then returned via line 35, 35 to line 35 and cell 31. Obviously operating the cell series of FIG. 3 in this manner will require that brine inlet and outlet lines would be supplied to feed brine to the cells either in series or in parallel as desired.

In this manner of operation clean mercury and amalgam are always introduced into the remaining cells and butter formation in the cells 32, 33 and 34 substantially reduced since purification takes place in the first cell in the series.

It is desirable to provide during the electrolysis taking place in the clean-up cell a control on the anode potential to prevent substantial oxidation of the mercury in the cell. This is preferably achieved by utilizing a reference electrode in a salt bridge system. In such a system, the reference electrode is electrically connected to a potentiometercontroller which operates to provide a signal to the rectifier controlling the operation of the cleaning cell. The reference electrode is also connected electrically to a sensing tip which is touching the surface of the mercury anode of the cleaning cell. In this manner any change in anode potential occurring on the anode surface is sensed, caused to provide a signal from the reference electrode to the control potentiometer which in turn signals the rectifier feeding the cleaning cell causing an appropriate voltage adjustment, either an increase or a decrease.

Attention is now directed to FIG. 5 which shows an anodic cleaning cell suitable for use in accordance with the present invention. In this cell there is shown a cell housing 100 having placed therein a steel cathode member 103. The cathode member is connected to a negative bus bar via lead 101 and has passing through it a sensing probe 104. Probe 104 at its tip, makes contact with the cell anode which comprises a mercury electrode 105. This electrode is in contact with a steel bottom member 106 which is electrically connected to the positive bus bar via lead 107.

Mercury is introduced into the cell via inlet line 108'.

The cathode is maintained in a spaced relationship from the mercury of the cell by means of a float member 109, the float member being constructed of a suitable nonconducting material typically, polyvinyl dichloride. Mercury is removed from the cell via outlet 110. In order to insure adequate cleaning of the mercury passing through the cell unit of FIG. 5, it is necessary that some control be placed on the cell to avoid the oxidation of the mercury utilized as the anode of the cell. In accordance with the instant invention a control is placed on this cell and generally comprises the arrangement shown in FIG. 6. As shown in FIG. 6 the control instrumentation involves a rectifier member 120 which is suitably connected to the anode cleaning cell leads 101 and 107. The sensing probe 104 is connected via salt bridge 121 to a reference cell 122. The utilization of the anodic cleaning cell depends in part on controlling the anodic potential of the mercury that is being cleaned. Thus, if the anode potential is too low, no cleaning will occur, and if the potential is too high, mercury will be anodically dissolved. Thus, in accordance with the present invention the anode potential of the cleaning cell is maintained between 0.3 to 0.6 volt, with reference to a standardsilver-silver chloride reference electrode. The control system as shown in FIG. 6 is such that the signal from the reference electrode 122 is sent through a voltage divider 123 and then into an electronic controller 124. The controller regulates the AC input to the rectifier 120 and alters the power input in this manner to the anodic cleaning cell in response to signals received from the controller 124. This system is such that the control of anode potential is within $0.025 volt.

As has been previously indicated, during the operation of a mercury cell film thickness builds upon the bottom plate of the cell to the point where hindered flow of mercury can occur. The present invention minimizes the formation of this film and provides a method whereby the film thickness can be controlled Within tolerable limits and optimized mercury fiow obtained. FIGS. 7 and 8 show graphically a comparison of the utilization of the instant invention as a control on the mercury thickness in an electrolytic cell of the type shown in US.

Ser. No. 410,579, filed Nov. 12, 1964, with an operation on the same cell with no control. It can be readily seen in FIG. 7 where no control was placed upon the mercury cell various film thicknesses were measured over a period of 28 days. It is to be noted that the film thickness began at a rather low level (slightly above .06 inch) and constantly built up during the 28-day period. Several flushing operations were required in order to maintain mercury flow throughout the cell during this period of operation. In FIG. 8 there is shown the measurement of various film thicknesses while the instant invention was being practiced on a cell which had already accumulated a substantial layer of sludge on the cell bottom. As can be readily appreciated, the film thickness was controlled and reduced substantially from the beginning of the operation to the end of the operation.

In the operation of a mercury cell, typically in the operation of a cell system of the type generally indicated by FIG. 1, a cleaning cell was employed which consisted of an epoxy resin lined steel box with a baffie 111 and two loop seals 108 and 110 for inlet and exit mercury streams, respectively, such as is shown in FIG. 5 of the attached drawings. The cell was 15 /2 inches long by 6 /2 inches wide and 7 /2 inches high. The mercury layer flowing through the length of the cell was the anode of the cell. The cathode was a sheet of expanded steel 7 inches long by 5 inches wide by A; inch thick attached to two polyvinyl dichloride sections which floated on the mercury surface. A sensing probe (Luggin tip) was used in measuring the anode potential and was mechanically connected to the cathode plate with a polypropylene fitting so that the top contacted the anode surface without dipping appreciably below the surface (less than 0.1 mm. below the surface).

A side stream taken from the primary denuder of a mercury cell was utilized as feed to the anodic cleaning cell. This was passed through a small auxiliary denuder prior to being fed to the cell. The auxiliary denuder minimized the size of the rectifier used for anodic cleaning cell and insured that the caustic soda losses from anodized amalgam was held to a minimum. The denuder was a 4-inch steel cyclindrical tube containing 6 inches of graphite packing. Sodium in the mercury feed to the cleaning cell contained less than 0.001 percent Na by weight. The operation of this cell was maintained such that an anode potential of between 0.3 to 0.6 volt was realized utilizing a reference electrode of silver-silver chloride. The silver-silver chloride reference electrode was a sleeve type electrode manufactured by the Fisher Scientific Company. The electronic controlling device was a Leeds Northrup Speed-a-max Recorder-Controller, type H, model 5. The rectifier was a silicon control rectifier (SCR) manufactured by the Sloan Instrument Corporation, Power Control model No. CRP-ZlS and the rectifier, a Sel-Rex model 10-60. Silicon control rectifier 124 controlled the AC input to the power rectifier of FIG. 6 and the instrumentation was capable of controlling the anodic cleaning cell during the operation at $0.025 volt. The cell was operated in this manner for a period of 28 days and a graph depicting the cleaning accomplished with the cell was made. This is depicted in FIG. 8 of the attached drawings and shows the substantial reduction in film thickness in the main cell accomplished utilizing this equipment.

While the invention has been described with reference to certain specific examples and illustrated embodiments, it is of course to be understood that the instant invention is not to be limited thereby except insofar as appears in the accompanying claims.

We claim:

1. In the operation of an alkali metal chlorine cell having a mercury cathode comprising removing mercury from said cell during operation, denuding said mercury and feeding so denuded mercury to an auxiliary electrolytic cell in which a small stream of denuded mercury removed from the main cell is utilized as a moving anode flowing through said auxiliary cell, and providing in said cell a suitable cathode, the improvement which comprises conducting electrolysis in the presence of an electrolyte in said auxiliary cell at a controlled anode potential sufficient to cause metal impurities present in the mercury feed to be removed therefrom by substantially preventing the mercury in said cell from anodically dissolving the electrolyte, said mercury anode potential being maintained with respect to a reference electrode which is an electrode other than said anode and cathode and removing from said auxiliary cell mercury substantially depleted of metal ion impurities.

2. The method of claim 1 wherein the small stream contains between 0.5 to 10 percent of the mercury flowing through said denuder.

3. The method of claim 1 wherein said anodic poten-' tial is maintained between 0.3 and 0.6 volt with reference to a standard silver-silver chloride reference electrode.

4. A method of purifying mercury removed from an electrolytic alkali metal chlorine cell having a flowing mercury cathode and containing metal ion impurities comprising denuding said mercury and introducing s denuded mercury to an electrolytic cell, utilizing said mercury as a moving mercury anode flowing through said cell, conducting electrolysis in said cell in the presence of said mercury at a controlled anode potential sufficient to cause metal ion impurities contained in the mercury to be removed therefrom but insufficient to cause dissolution of mercury in the cell electrolyte, said anodic potential being controlled at the anode surface with respect to a reference electrode which is an electrode other than said cathode and anode, removing from the cell a mercury substantially depleted in metal ion impurities and returning said purified mercury to the flowing mercury cathode of the main electrolytic cell.

5. In the operation of an alkali metal chlorine cell having a flowing mercury cathode wherein mercury is passed through a first cell producing elemental chlorine and a sodium amalgam and the sodium amalgam is utilized in a series of subsequent cells as the cathode of said cells for the purpose of providing a concentrated sodium amalgam from the last cell in the series while elemental chlorine is produced in each of the cells, the improvement comprising removing a small side stream from one of the series connected cells, denuding said side stream to provide a mercury stream containing metal ion impurities, passing said mercury stream to an auxiliary electrolytic cell wherein said mercury is an anode, providing in said auxiliary electrolytic cell a cathode member and a suitable electrolyte, electrolyzing said electrolyte in the auxiliary cell at an anodic potential sufficient to remove metal ion impurities from the mercury fed thereto while preventing any substantial dissolution of mercury on the electrolyte, said anodic potential being controlled with reference to a reference electrode independent of said cleaning cell, continuously removing from the cell mercury substantially depleted of its metal ion content, returning said mercury to the series cell operation for the production of further quantities of elemental chlorine and to thereby control the operation of said cell series by minimizing the formation of the cell butters.

6. A method of controlling the metal ion impurity level during the electrolysis of alkali metal chloride solutions in a cell containing a flowing mercury cathode comprising removing sodium amalgam from said cell, introducing said amalgam to a denuder to convert said amalgam to sodium hydroxide and elemental mercury, feeding said mercury to an auxiliary electrolytic cell wherein said mercury is utilized as a flowing mercury anode, providing in said cell a suitable electrode as cathode and electrolyte, electrolyzing said electrolyte in said auxiliary cell at an anodic potential sufficient to remove metal ion impurities contained in said mercury while substantially preventing the solubilization of the mercury utilized as the anode, controlling the power input to said auxiliary cell at a voltage sufficient to accomplish this purpose by utilizing a reference electrode member electrically connected to the power rectifier input to said auxiliary cell to maintain the anode potential within a range capable of removing the metal ion impurities of the mercury while preventing the mercury from solubilizing in the electrolyte and removing continuously from said cell a mercury substantially depleted of its metal ion content.

7. In the operation of an alkali metal chlorine cell having a flowing mercury cathode and a denuder associated therewith wherein amalgam removed from the cell is denuded to provide alkali metal hydroxide and mercury and the mercury is recirculated to an alkali metal chlorine cell, the improvement comprising removing a portion of the denuded mercury being recirculated to the cell and feeding it to an auxiliary cell, utilizing the so removed portion of denuded mercury as the anode of said auxiliary cell, providing said auxiliary cell with a cathode and electrolyte, electrolyzing the electrolyte in said auxiliary cell at an anodic potential suflicient to cause metal ion impurities in said mercury to be removed therefrom while preventing any substantial solubilization of the mercury anode, controllably maintaining said anodic potential with reference to a reference electrode which is an electrode other than said anode or cathode, and removing from the auxiliary cell mercury substantially depleted of its metal ion impurities, recirculating the mercury substantially depleted of its metal ion impurities to an alkali metal chlorine cell.

8. In the operation of an alkali metal chlorine cell having a flowing mercury cathode in which sodium amalgam is continuously removed from said cell and passed to a denuder wherein a small stream of the mercury removed from said denuder and passed to an anodic cleaning cell comprising as its anode mercury removed from the denuder and having a cathode and containing electrolyte, electrolyzing the electrolyte in the cleaning cell to remove metal ions in the mercury, removing from said cell mercury substantially depleted of its metal ion content and returning so depleted mercury to an alkali metal chlorine cell, the improvement which comprises conducting the electrolysis in the anodic cleaning cell at an anodic potential on the mercury anode surface suflicient to remove metal ion impurities in the mercury but small enough to prevent substantial mercury from oxidizing into said electrolyte, which anodic potential is automatically controlled with reference to a standard reference electrode other than the anode or cathode of said cleaning cell. 55

References Cited UNITED STATES PATENTS 2,067,361 1/1937 Vivian 204140 2,316,685 4/1943 Gardiner 204-99 3,091,579 5/1963 Basilevsky 20499 3,364,128 1/1968 Williston et al. 204

FOREIGN PATENTS 6 665,225 1/1952 Great Britain 20499 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. Cl. X.R. 

